Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving member. The ink may be stored in reservoirs that are located within cartridges installed in the printer. Such ink may be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink may be in the form of pellets, ink sticks, granules, pastilles, or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device, which melts the solid ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. Other inkjet printers use gel ink. Gel ink is provided in gelatinous form, which is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead. Once the melted solid ink or the gel ink is ejected onto the image receiving member, the ink returns to a solid, but malleable form, in the case of melted solid ink, and to gelatinous state, in the case of gel ink.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving surface to form an ink image during printing. The image receiving surface may be the surface of a continuous web of recording media, a series of media sheets, or the image receiving surface may be a rotating surface, such as the surface of a rotating print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an aperture, usually called a nozzle, in a faceplate of the printhead. The actuators expel an ink drop in response to an electrical signal, sometimes called a firing signal. The magnitude, or voltage level, of the firing signals affects the amount of ink ejected in an ink drop. The firing signal is generated by a printhead controller with reference to image data. A print engine in an inkjet printer processes the image data to identify the inkjets in the printheads of the printer that must be operated to eject a pattern of ink drops at particular locations on the image receiving surface to form an ink image corresponding to the image data. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to electronic image data.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads are registered with reference to the image receiving surface and with the other printheads in the printer. Registration of printheads refers to a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the relative positions of the printheads with reference to the imaging surface and with reference to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with one another across a width of the image receiving member and that all of the inkjets in the printhead are operational.
The setting of a proper gap is important in a printer designed to accept a variety of imaging surfaces including those having different thickness or those that have a tendency to wrinkle. A thin layer of polytetrafluoroethylene (PTFE) is disposed on the printheads to provide accurate imaging. The PTFE layer controls the drooling pressure at the orifices of the printhead and should not be touched or burnished by wax or paper. If the PTFE is damaged, the drooling pressure can drop which in turn can alter the path of ejected ink drops as well as provide drooling or dripping of ink during printing resulting from weak and missing inkjets.
A continuous web of recording media is transported from a paper manufacturer to an end user as a roll of material. The outer edges of the web roll can be adversely affected during transportation due to shipping and handling. For instance either of the edges, which are often exposed, can be adversely affected. When the damaged edges move through the printer, the damaged edge can strike the face of the printhead due to the small gap between the surface of the recording media and the face of the printhead. These damaged edges can adversely impact the printheads. To reduce the risk of contact with the printheads, an operator typically cuts off about one inch of the outer edge of the web roll before mounting the web roll to the machine. While removing the outer one inch of material from the edges of the web roll can eliminate or substantially reduce damage to the printheads, this procedure is not desirable since both material and time are wasted.
In addition to edge defects being present in the web media, other defects can also occur, such as wrinkles or troughs. Wrinkles can occur when adjacent areas of the web media are forced closer together under stress. Wrinkles can result from a variety of sources, but typically occur during manufacture of the web media or preparing the web media for transport on a roll. Wrinkles oftentimes run lengthwise or along the process direction in the direction that the web media is transported through the printer. Troughs can also occur and are characterized as any long depression or channel. Troughs are often found to include a recessed area with respect to the rest of the surrounding web media, where the adjacent areas of the rest of the web media can be slightly elevated with respect to the trough.
Other web media defects not characterized as edge damage, wrinkles or troughs, can also be present in the recording media, such as foreign matter fixed in or held by the web media. Consequently, any change to the surface of the web media which deviates from a generally flat or smooth surface of the web can be considered as a defect. It is desirable therefore to prevent defects in the web media from reaching the printhead to reduce or prevent damage to the printhead.
While image quality suffers from defects in the recording media, poor image quality can also result from one or more printheads depositing ink at incorrect locations on the web media. For instance, if one of the rollers transporting the web media through the printer is out of round or the axis of rotation of the roller about a supporting rotational shaft is incorrect, improper registration of ink drops or pixel can occur. Consequently, it is also desirable to insure that defective transport rollers do not contribute to imaging errors.
The gap, or distance, between a printhead and the imaging surface is controlled to optimize the imaging process. If the gap is too small, burnishing of the printheads can occur when the image receiving surface contacts the face of the printheads. Burnishing not only can reduce the life of the printheads, but can produce poor image quality and increased downtime of the printer during maintenance. In one embodiment, if the gap between the printhead and the image receiving surface is less than about 0.6 millimeters, damage to the printhead, including burnishing, can occur.